The polymerization of lower .alpha.-olefins to produce thermoplastic polymers is an industry of substantial commercial importance. The polymeric products of such a process, e.g., polypropylene, polyethylene and ethylene/propylene copolymers, are important materials of commerce because of the relatively low cost of the polymers and the desirable properties they possess. The polymerization of ethylene is relatively uncomplicated because the polyethylene polymer exists in only one steric form. Higher .alpha.-olefins such as propylene form polymers of several steric types because of the pendant alkyl group of the olefin monomer. The cost and value of polypropylene, for example, will be greatly influenced by the steric form in which the polymer is produced. Most commercial polypropylene is crystalline and highly stereoregular and is usually isotactic. Polypropylene which is not stereoregular is termed atactic and is not crystalline. This amorphous polymer is less desirable and, if present in substantial quantities, must usually be removed as by extraction before the polypropylene will have commercially attractive properties. In recent commercial polypropylene production, it is virtually mandatory for economic reasons to employ polymerization catalyst which is sufficiently active and produces a highly stereoregular product so that polypropylene of acceptable properties will be produced without the need for extraction or deashing steps.
The production of such an active, stereoregular catalyst is frequently a rather complicated process with much of the complexity being encountered during the production of what is conventionally termed the olefin polymerization procatalyst. This catalyst precursor is frequently a titanium-containing solid and often contains moieties of magnesium and halide, particularly chloride. For polyethylene production, the procatalyst is frequently a vanadium-containing solid. Such procatalysts are described in numerous patents and other references and vary in chemical character, depending upon the particular catalyst desired. One class of procatalyst results from the reaction of a magnesium compound, often a magnesium alkoxide compound, with a tetravalent titanium halide in the presence of a halohydrocarbon reaction diluent and an electron donor which is often an alkyl ester of an aromatic monocarboxylic or dicarboxylic acid.
The procatalyst is generally a solid material and is easily separated from the media of its production. The remaining waste product is a liquid material and contains at least some of unreacted heavy metal halide, halohydrocarbon, e.g., chlorobenzene, unreacted electron donor, and a wide array of haloalkoxide compounds or complexes thereof with other chloroalkoxide compounds or aromatic esters.
This waste product from such procatalyst production presents a substantial disposal problem which also adversely affects the economy of the polymerization process. It is an advantage to be able to separate the components of such a waste stream and to recover for reuse the more valuable components of the product such as titanium tetrachloride and the halohydrocarbon reaction diluent.
One such method of component separation is described by Potter et al., U.S. Pat. No. 5,242,549. This references provides for the separation of waste product components by a method wherein a separation solvent is added to the waste product, and the liquid components are separated by distillation. The resulting waste stream of that patent comprises titanium compounds, such as titanium alkoxides and chloroalkoxides and complexes thereof, dissolved in the separation solvent. The separation solvent is typically an aromatic halohydrocarbon such as chlorobenzenes and chlorotoluenes. A great economic advantage would be realized if the metal compounds could be removed from the separation solvent as solids for disposal, and the separation solvent further purified to be reused in the distillation process.